1998 Mazda Truck overheating slightly

I just bought my truck for $600 and fully rebuilt the engine. I got new themostat replaced all cooling hoses drove it about 146miles runing to cool so I took themostat out to see it was stuck open it wasent put it back it now when driveing it seems to get a little to hot im worried the engine is going to blow up on me can you give me some tips on fixing my problem?